For the purpose of conservation of resources, it is an extremely important problem in the present-day society to recover reusable resources from wastes, apply appropriate treatment and again make them useful, namely to recycle resources. Above all, plastics are made of, as a major raw material, petroleum which is limited resources and when treated as a waste, are liable to cause a lot of environmental problems, and importance for recycling them is large. In waste plastics including polyethylene terephthalate-made bottles, whose sorted collection is relatively advanced, a proportion in which materials used as a container for foodstuffs, cosmetics, detergents, and the like account for is large. In these polyethylene terephthalate-made bottles or the like, in major cases, polyethylene and/or polypropylene which is a plastic with a low specific gravity is used in a cap portion, whereas polyethylene and polystyrene are used for a label or the like. In recycling, such plastics are required to be efficiently sorted with high precision. In advancing the recycle activity, there is desired a technology capable of increasing such sorting precision while increasing the sorting precision and recovery.
As a method for sorting a plastic composite raw material of plastics into a single raw material, a method for the separation by sorting by specific gravity utilizing a gas flow is known (see, for example, Patent Document 1). However, in the gas, a purity of the sorted plastic is limited to about 60 to 70%, and it is difficult to achieve a recycle method to be employed for the same application as in virgin materials. Also, as a sorting method of aplastic having a specific gravity exceeding 1.0 and a plastic having a specific gravity of less than 1.0, there is a method in which plastics are charged in an open water tank and separated into a precipitated plastic and a floated plastic (see, for example, Patent Documents 2 and 3). According to this method, in the case where polystyrene and a polyester, both of which have a specific gravity exceeding 1.0 coexist, it is difficult to sort the both from each other.
Furthermore, there is proposed a method of feeding a water slurry of waste plastics into a center of a cylindrical classifier filled with a liquid and provided with top and lower outlets and recovering a plastic with a low specific gravity from the upper outlet and a plastic with a high specific gravity from the lower outlet by an upward flow and a downward flow, respectively while revolving with a swirling flow (see, for example, Patent Document 4). However, in this method, since the downward flow of the liquid component exists, it is actually difficult to increase chiefly a purity of the plastic with a high specific gravity to 99% or more by the liquid flow. Also, since the rate of the liquid flow falls within a low flow rate range, a problem that very large equipment is required for achieving treatment on a large scale and other problems are involved. Besides, there is also proposed a method for sorting a plastic of a single raw material from a water slurry of waste plastics by a liquid specific gravity separator having a liquid cyclone shape (see, for example, Patent Document 5). However, in major cases, such equipment is generally low in the treatable amount and is unable to respond to the treatment amount on a large scale. For that reason, in the case where the treatment amount increases, it is necessary to set up a number of separators, and there is involved a defect that it cannot be said that this is economically effective.    (Patent Document 1) JP-A-6-63941    (Patent Document 2) JP-A-48-100760    (Patent Document 3) JP-A-4-326955    (Patent Document 4) JP-A-9-15417    (Patent Document 5) JP-A-2000-288422